For a number of years, much research has been carried out to discover organic, metallic or semiconductive radicals able to be used in magnetometry (EPR) and in particular ionic systems have been developed, such as fluoranthene salts and tetrathiafulyalene-tetracyanoquinodimethane, described by E. Dormann et al in the Journal of Magnetism and Magnetic Materials, 54-57, (1986), p. 1315-1316.
The use of these magnetometry compounds suffers from certain drawbacks. In fact, the salts of radical ions belonging to the fluoranthene family or (TTF-CNQ) are systems with several compounds and the problems linked to their unpredictable stoichiometric qualities and lack of chemical and thermal stability are inevitable. In addition, the electric conduction properties of these substances disturb the magnetic field measurements via the skin effect.
Also, researches have been carried out so as to find other more high-performing materials.
Amongst the materials able to offer properties suitable for use in magnetometry, radical-like phthalocyanines, such as lithium phthalocyanine, have been considered, as described by Turek et al in the publication Solid State Communications, Vol. 63, n 8, p. 741-744, 1987. This radical-like lithium phthalocyanine may be prepared by potentiostatic electrochemical oxidation of dilithiated phthalocyanine, as described by Sujimoto et al in the publication J. Chem. Soc. Chem. Commun., 1986, p. 962-963.
The radical-like lithium phthalocyanine seemed suitable for carrying out magnetic measurements, but the signal obtained with this compound is inadequate to justify its industrial application. Moreover, the radical-like lithium phthalocyanine is extremely sensitive to the action of oxygen, as described by Turek et al in the aforesaid document, which leads to a widening of the EPR line.
Furthermore, the preparation of the radical-like lithium phthalocyanine by potentiostatic electrochemical oxidation does not make it possible to obtain crystals having satisfactory properties. In fact, the generation of lithium phthalocyanine crystals starts at a maximum intensity value in order to reduce subsequently. This results in a rapid non-reproducible growth of the radical-like lithium phthalocyanine in the form of a powder or very small crystals, which comprises defects in the best of cases.